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<h1 id="firstHeading" class="firstHeading">显式（全）模板特化</h1>
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<tr class="t-nv"><td colspan="5"> <strong class="selflink">显式（全）特化</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="partial_specialization.html" title="cpp/language/partial specialization">部分特化</a> </td></tr>
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<p>允许对给定的模板实参集定制模板代码。
</p>
<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>template</b></code> <code><b>&lt;&gt;</b></code> <span class="t-spar">声明</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<p>以下任何一项均可以完全特化：
</p>
<ol><li> <a href="function_template.html" title="cpp/language/function template">函数模板</a>
</li><li> <a href="class_template.html" title="cpp/language/class template">类模板</a>
</li><li> <span class="t-mark-rev t-since-cxx14">(C++14 起)</span><a href="variable_template.html" title="cpp/language/variable template">变量模板</a>
</li><li> 类模板的<a href="member_functions.html" title="cpp/language/member functions">成员函数</a>
</li><li> 类模板的<a href="static.html" title="cpp/language/static">静态数据成员</a>
</li><li> 类模板的<a href="nested_classes.html" title="cpp/language/nested types">成员类</a>
</li><li> 类模板的成员<a href="enum.html" title="cpp/language/enum">枚举</a>
</li><li> 类或类模板的<a href="member_template.html" title="cpp/language/member template">成员类模板</a>
</li><li> 类或类模板的<a href="member_template.html#.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0.E6.A8.A1.E6.9D.BF" title="cpp/language/member template">成员函数模板</a>
</li></ol>
<p>例如， 
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>   <span class="co1">// 主模板</span>
<span class="kw1">struct</span> is_void <span class="sy4">:</span> <a href="../types/integral_constant.html"><span class="kw650">std::<span class="me2">false_type</span></span></a>
<span class="br0">{</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>  <span class="co1">// 对 T = void 显式特化</span>
<span class="kw1">struct</span> is_void<span class="sy1">&lt;</span><span class="kw4">void</span><span class="sy1">&gt;</span> <span class="sy4">:</span> <a href="../types/integral_constant.html"><span class="kw649">std::<span class="me2">true_type</span></span></a>
<span class="br0">{</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="co1">// 对于任何非 void 的类型 T，该类均派生自 false_type</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> is_void<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> 
    <span class="co1">// 但当 T 为 void 时类派生自 true_type</span>
    <a href="../io/cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> is_void<span class="sy1">&lt;</span><span class="kw4">void</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">value</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<h3><span class="mw-headline" id=".E7.BB.86.E8.8A.82">细节</span></h3>
<p>显式特化可以在可以定义其主模板的任何作用域中声明（这可以不同于定义其主模板定义的作用域；例如同<a href="member_template.html" title="cpp/language/member template">成员模板</a>的类外特化）。显式特化必须出现在非特化模板声明后。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">namespace</span> N <span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> X <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 主模板</span>
    <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">class</span> X<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 同命名空间中的特化</span>
 
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> Y <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 主模板</span>
    <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">class</span> Y<span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// 对 double 特化的前置声明</span>
<span class="br0">}</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">class</span> N<span class="sy4">::</span><span class="me2">Y</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span> <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// OK：同命名空间中的特化</span></pre></div></div>
<p>特化必须在第一条导致隐式实例化的使用之前，在每个发生这种使用的翻译单元中声明：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">class</span> String <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> Array <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> sort<span class="br0">(</span>Array<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v<span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span> <span class="co1">// 主模板</span>
 
<span class="kw4">void</span> f<span class="br0">(</span>Array<span class="sy1">&lt;</span>String<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v<span class="br0">)</span> <span class="br0">{</span>
    sort<span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 隐式实例化 sort(Array&lt;String&gt;&amp;), </span>
<span class="br0">}</span>            <span class="co1">// 使用初等模板 sort()</span>
 
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>  <span class="co1">// 错误：sort(Array&lt;String&gt;) 的显式特化出现在隐式实例化之后</span>
<span class="kw4">void</span> sort<span class="sy1">&lt;</span>String<span class="sy1">&gt;</span><span class="br0">(</span>Array<span class="sy1">&lt;</span>String<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v<span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>仅声明但未定义的模板特化，可像其他<a href="incomplete_type.html#.E4.B8.8D.E5.AE.8C.E6.95.B4.E7.B1.BB.E5.9E.8B" title="cpp/language/type">不完整类型</a>一样使用（例如可以使用到它的指针和引用）
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> X<span class="sy4">;</span> <span class="co1">// 主模板</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">class</span> X<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// 特化（声明，不定义）</span>
X<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// OK：指向不完整类型的指针</span>
X<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> x<span class="sy4">;</span> <span class="co1">// 错误：不完整类型的对象</span></pre></div></div>
<h3><span class="mw-headline" id=".E5.87.BD.E6.95.B0.E6.A8.A1.E6.9D.BF.E7.9A.84.E6.98.BE.E5.BC.8F.E7.89.B9.E5.8C.96">函数模板的显式特化</span></h3>
<p>当特化函数模板时，若<a href="template_argument_deduction.html" title="cpp/language/template argument deduction">模板实参推导</a>能从函数实参予以提供，则可忽略其实参：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw1">class</span> Array <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> sort<span class="br0">(</span>Array<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy3">&amp;</span> v<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 主模板</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> sort<span class="br0">(</span>Array<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy3">&amp;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 对 T = int 的特化</span>
<span class="co1">// 不需要写为</span>
<span class="co1">// template&lt;&gt; void sort&lt;int&gt;(Array&lt;int&gt;&amp;);</span></pre></div></div>
<p>与某个特化带有相同名字和相同形参列表的函数不是特化（见<a href="function_template.html" title="cpp/language/function template">函数模板</a>中的重载）
</p><p>仅当函数模板的显式特化声明为带 <a href="inline.html" title="cpp/language/inline">inline 说明符</a>（或定义为弃置）时，它是内联函数，主模板是否为内联对其没有影响。
</p><p>不能在函数模板，成员函数模板，以及在隐式实例化类时的类模板的成员函数的显式特化中指定<a href="default_arguments.html" title="cpp/language/default arguments">默认函数实参</a>。
</p><p>显式特化不能是<a href="friend.html" title="cpp/language/friend">友元声明</a>。
</p><p>若主模板具有并非 <code>noexcept(false)</code> 的异常说明，则显式特化必须具有兼容的异常说明。
</p>
<h3><span class="mw-headline" id=".E7.89.B9.E5.8C.96.E7.9A.84.E6.88.90.E5.91.98">特化的成员</span></h3>
<p>在类体外定义显式特化的类模板的成员时，不使用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1">&lt;&gt;</span></span></span> 语法，除非它是某个被特化为类模板的显式特化的成员类模板的成员，因为其他情况下，语法会要求这种定义以嵌套模板所要求的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span>形参<span class="sy1">&gt;</span></span></span> 开始
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span> <span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw1">struct</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>  <span class="co1">// 成员类 </span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw1">struct</span> C <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 成员类模板</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// 特化</span>
<span class="kw1">struct</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span> <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 特化的成员函数</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// template&lt;&gt; 不用于特化的成员</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* ... */</span> <span class="br0">}</span>
 
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// 成员类的特化</span>
<span class="kw1">struct</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span> <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// template&lt;&gt; 亦不用于特化的成员类的成员</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* ... */</span> <span class="br0">}</span>
 
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// 成员类模板的的定义</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw1">struct</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">C</span> <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// template&lt;&gt; 在作为类模板定义显式特化的成员类模板时使用</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> U<span class="sy1">&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">C</span><span class="sy1">&lt;</span>U<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* ... */</span> <span class="br0">}</span></pre></div></div>
<p><br>
模板的静态数据成员的显式特化，若其声明包含初始化器则为定义；否则，它是声明。这些定义对于默认初始化必须用花括号：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;&gt;</span> X Q<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">x</span><span class="sy4">;</span> <span class="co1">// 静态成员的声明</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> X Q<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">x</span> <span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 错误：函数声明</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> X Q<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">x</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 静态成员的默认初始化定义</span></pre></div></div>
<p>类模板的成员或成员模板可对于类模板的隐式实例化显式特化，即使成员或成员模板定义于类模板定义中。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span>
<span class="kw1">struct</span> A <span class="br0">{</span>
    <span class="kw4">void</span> f<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 成员，声明于主模板中</span>
    <span class="kw4">void</span> h<span class="br0">(</span>T<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="co1">// 成员，定义于主模板中</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X1<span class="sy1">&gt;</span> <span class="kw4">void</span> g1<span class="br0">(</span>T, X1<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 成员模板</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X2<span class="sy1">&gt;</span> <span class="kw4">void</span> g2<span class="br0">(</span>T, X2<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 成员模板</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// 成员的特化</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">f</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co1">// 成员特化 OK，即使定义于类中</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">h</span><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
 
<span class="co1">// 类外成员模板定义</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X1<span class="sy1">&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span>T<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g1</span><span class="br0">(</span>T, X1<span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
 
<span class="co1">// 成员模板特化</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X1<span class="sy1">&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g1</span><span class="br0">(</span><span class="kw4">int</span>, X1<span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// 成员模板特化</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g2</span><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">char</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 对于 X2 = char</span>
<span class="co1">// 同上，用模板实参推导 (X1 = char)</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> 
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">g1</span><span class="br0">(</span><span class="kw4">int</span>, <span class="kw4">char</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>成员或成员模板可嵌套于多个外围类模板中。在这种成员的显式特化中，对每个显式特化的外围类模板都有一个 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;&gt;</span></span></span>。
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T1<span class="sy1">&gt;</span> <span class="kw1">class</span> A <span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T2<span class="sy1">&gt;</span> <span class="kw1">class</span> B <span class="br0">{</span>
        <span class="kw4">void</span> mf<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
    <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">class</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">mf</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span></pre></div></div>
<p>在这种嵌套声明中，某些层次可保留不特化（但若其外围类未被特化，则不能特化类成员模板）。对于每个这种层次，声明需要 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1">&lt;</span>实参<span class="sy1">&gt;</span></span></span>，因为这种特化自身也是模板：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T1<span class="sy1">&gt;</span> <span class="kw1">class</span> A <span class="br0">{</span>
    <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T2<span class="sy1">&gt;</span> <span class="kw1">class</span> B <span class="br0">{</span>
        <span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T3<span class="sy1">&gt;</span> <span class="kw4">void</span> mf1<span class="br0">(</span>T3<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 成员模板</span>
        <span class="kw4">void</span> mf2<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 非模板成员</span>
     <span class="br0">}</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// 特化</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// 对于特化的 A</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> X<span class="sy1">&gt;</span> <span class="co1">// 对于未特化的 B</span>
<span class="kw1">class</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span> <span class="br0">{</span>
    <span class="kw1">template</span> <span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="kw4">void</span> mf1<span class="br0">(</span>T<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// 特化</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// 对于特化的 A</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="co1">// 对于特化的 B</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> T<span class="sy1">&gt;</span> <span class="co1">// 对于未特化的 mf1</span>
<span class="kw4">void</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">mf1</span><span class="br0">(</span>T t<span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span>
 
<span class="co1">// 错误：B&lt;double&gt; 被特化而且是成员模板，故其外围的 A 也必须特化</span>
<span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">class</span> Y<span class="sy1">&gt;</span>
<span class="kw1">template</span><span class="sy1">&lt;&gt;</span> <span class="kw4">void</span> A<span class="sy1">&lt;</span>Y<span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">B</span><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">mf2</span><span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <span class="br0">}</span></pre></div></div>
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> 应用于
</th>
<th> 出版时的行为
</th>
<th> 正确行为
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://wg21.cmeerw.net/cwg/issue727">CWG 727</a>
</td>
<td> C++14
</td>
<td> 不允许在类作用域的全特化，即使允许部分特化
</td>
<td> 允许在任何作用域的全特化
</td></tr></table>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<ul><li> <a href="templates.html" title="cpp/language/templates">模板</a>
</li><li> <a href="class_template.html" title="cpp/language/class template">类模板</a>
</li><li> <a href="function_template.html" title="cpp/language/function template">函数模板</a>
</li><li> <a href="partial_specialization.html" title="cpp/language/partial specialization">部分特化</a>
</li></ul>

<!-- 
NewPP limit report
Preprocessor visited node count: 1466/1000000
Preprocessor generated node count: 5196/1000000
Post‐expand include size: 36385/2097152 bytes
Template argument size: 9507/2097152 bytes
Highest expansion depth: 15/40
Expensive parser function count: 0/100
-->

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